A variety of technologies for determining a color mode of a target image have been suggested.
For example, an image processing apparatus disclosed in JP-A-2007-251740 converts image data, which is input in a page unit from an image scanning unit and is resolved into three colors of R, G and B, into bitmap data of four colors of yellow (Y), cyan (C), magenta (M) and black (K). Subsequently, the apparatus counts a number of color pixels of a whole page for each color and compares the counted number of color pixels with a threshold value. When at least one of the numbers of the color pixels for the three colors is larger than the threshold value, the apparatus determines that the image data is a color image. Meanwhile, when all of the number of color pixels is the threshold value or smaller, the apparatus determines that the image data is a black-and-white image.
However, according to the image processing apparatus disclosed in JP-A-2007-251740, the determination result is influenced by a ratio of the number of color pixels to a total number of pixels of a whole page. Therefore, even when the image data is a color image, the image data may be determined as a black-and-white image.
For example, as shown in FIG. 10, a pixel ratio of the number of color pixels 15000 to the total number of pixels 100000 of a whole page is 15% in an upper color image 100. When the threshold value for identifying a color mode is 2% or larger of the total number of pixels, the pixel ratio 15% is larger than the threshold value 2%. Accordingly, the upper color image 100 is determined as a color image. Meanwhile, in a lower color image 101 obtained by reducing the upper color image 100, a pixel ratio of the number of color pixels 1500 to the total number of pixels 100000 of a whole page is 1.5%. Accordingly, when the threshold value for identifying a color mode is 2% of the total number of pixels, the lower color image 101 is not determined as a color image, but is determined as a black-and-white image.